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Why is the inlet cone on some subsonic engines conical (sharp)?
An inlet cone, as far as I know, is a component on an engine to guide the intake air smoothly into the compressor stage of a jet turbofan engine, it performs as a fairing(aerodynamic covering)of the axis on which the intake fan is installed, to improve the aerodynamic efficiency. *The engines are all on subsonic airliners.
However, I found that on some modern high bypass turbofan engines, this cone(fairing)is in a pointed, sharp, conical design that seems to only appear on supersonic aircrafts, as a rounded design is optimized for subsonic flight. For example, in the images below:
While on some turbofan engines, this cone(fairing)is in a smooth, rounded design, very much like it would have been used on the nose design of the aircraft. For example, the engines on a MD-80:
Why do those modern engines have this conical sharp intake cone(or fairing)design? Is this because of the air acceleration above the speed of sound when intaking air at cruising speeds?

This is not an inlet cone as found on supersonic aircraft engines. This is a spinner used on subsonic turbofan engines, both the rounded and pointed one. Older engines used rounded ones; the JT8D in your question was first designed in 1963. Later engines use pointed ones. There are three purposes:

The spinner provides a smooth flow path for air entering the engine. Both shapes can do this well.

The shape minimizes ice accumulation so anti icing is not necessary.

The conical and elliptical shape helps to deflect FOD away from the engine core. Pointed spinners are better at remaining intact after impact, as from a bird strike. FAA testing for bird ingestion gets only more rigorous, as in this proposal.

Thank you for the answer. However, I found that the engines(PW-1100)on the later released A320-neo actually have rounded spinners. As in your answer, this shape is not the optimal design, and this newly-designed engine shouldn't be using such spinners. What is the purpose of still using them?

@RichardM This is a design choice. I gave you the main considerations which neither choice would fail to satisfy when done correctly. Older GE and Rolls engines like the CF6 and Avon used rounded spinners but newer engines from these same manufacturers use pointed. Pratt generally has stuck with rounded except for its v2500 IAE joint venture engine, apparently deciding that there was not enough difference to make a change.

To elaborate on Pilothead's answer's point #2:
A conical spinner accumulates ice less than a rounded spinner because it has a much smaller stagnation point (well, nearly-stagnant area).  However, it also sheds more ice into the core, at least for the B737 (italics are my addition):

The CFM56-7 spinner has a unique conelliptical profile. The first 737-3/400's had a conical (sharp pointed) spinner but these tended to shed ice into the core. This was one of the reasons for the early limitation of minimum 45% N1 in icing conditions which made descent management quite difficult. They were later replaced with elliptical (round nosed) spinners which succeeded in deflecting the ice away from the core, but because of their larger stagnation point, were more prone to picking up ice in the first place. The conelliptical [pointy, then rounded farther aft] spinner of the [737-NG]'s neatly solves both problems.


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Expected behaviour in Linkloss scenarios - UAVs
Imagine a remoted piloted aircraft like MQ-9 being controlled by pilot on the ground.
I was wondering what is the expected behaviour of the aircraft when the connection with the pilot is lost?
Does the RPA keep flying with the old commands defined by the pilot before the connection is interrupted? Does it fly a defined trajectory while being tracked by the ATC?, or do they fly towards a preplanned safe route? (autoland?).
If the second solution applies, the RPA would need a fast algorithm to calculate the path (while avoiding obstacles, restricted zones, ...). Any idea how this is done and which algorithm they might implement?

Just FYI you wouldn't really use the term "algorithm" there in a programming sense; also it's not especially "fast" (all software is "fast" :) )

I would say - Any and all of the above, depending on circumstances.
Upon Loss of Signal:

Hold in place (Circle for XX minutes, to see if signal returns)
Return To Base (RTB)  right now
Go somewhere else and land
Do some other mission, then RTB
Do something else

And also, location dependent. What happens near home base in Nevada would be different than what happens over hostile territory.
There isn't a singular or "expected" default of "what happens". It all comes down to how it is programmed.
Even a little $300 consumer UAV is programmable as to a LOS event.

Every product/project will implement the approach that they think best, given that it is approved by the authorities.
For my current project, we still have to get the permit to fly by EASA, but we're going for an autoland.
The algorithm to compute the path has been developed by me, so it does not really have a name. Moreover, rather than being simply an algorithm, I would say it is a full fledged program, given the amount of possible input and scenarios.
Being a very early demonstrator/proof-of-concept kind of thing, we're not going to have obstacles in our testing area.

This differs how the drone is specifically programmed, but here are the modes I know have been used.

Continue flight path: Often, the operator pre-specifies a mission with the help of a series of waypoints. If the telemetry-link to the drone is lost but it still has a valid navigation solution (e.g. GPS signal) the drone will simply continue this pre-planned mission. At the end of this flight-path, a holding pattern can often be introduced, on which the drone waits for a "go-ahead" signal (of course only when it has a valid radio signal). The reasoning is to not endanger your airbase or recovery site. If everything goes wrong the drone would crash at the location of that holding pattern.
Hold pattern on location: In certain flight modes, or when no waypoints were planned, often the only course of action is to enter a holding pattern at the current location. This also is true, when the drone loses all its communication signals including GPS signal. Because in that case the drone doesn't know where it is, nor can it receive any information from the outside, this is the only option (besides crashing). The now infamous
Beast of Kandahar was captured by the Iranias with GPS-spoofing, such that the RQ-170 entered a holding pattern until it ran out of fuel and glided to the ground (at least that is the speculation of what happend).
Dynamic Return to Home: This is the case you mentioned about planning a way home. The drone would either re-track its flight path back to where it launched, or it would dynamically plan a path home. This case is often more trivial then you think, because on the one hand you have plenty of time to compute this flight path, and you can simply high enough up, such that the airspace can be considered obstacle free. In that case you only need some elevation map to ensure that you do not controlled fly into the terrain. The downside to this is that this drone who is trying to return home, does not react to air traffic, for this reason it is often deemed more safe to simply ditch the drone. A lot of research exists in which safe dynamic planning is attempted, but a big problem is always environment perception. A nice paper imo on this topic can be found here (no affiliation). At the heart of a lot of these dynamic planning algorithms is the well-known and relatively simple A* algorithm.

I disagree that a hold pattern is the only option if you lose all radio signals including GPS. You can still use magnetic compass and barometric altimeter to navigate. It's not accurate, but good enough to get back to a place where you had GPS.

Rainer, I believe U_flow was simply stating some of the typical choices which have been used.  There's no "only option" on anything, it's just what the operators and makers choose to program in.  The programming (for what happens when connection is lost) is in itself trivial, you're just telling it to do things you've already programmed (in the other billion lines of software, you know?)

@RainerP. this is why I used "often" :D. A drone, as you propose, which stupidly flies in one direction is in most cases considered to be a risk to itself and other air traffic participants, because of course it has no means to detect other aircraft and therefore cannot evade. Especially if the drone in question suffered a malfunction instead of a electronic warfare attack often your safest option is to simply enter a hold pattern on location... There are other options, for example dead reckoning for navigation, but again that is often a design trade-off between safety and reliability

@U_flow - I agree with your reasoning, yet I think it is stupid to loiter in place. The most common reason for "loss of signal" is that the drone flew out of range of the transmitter. The correct remedy is to turn around immediately and fly back to the place where you came from and where you still had some signal. For a military drone, even more so. If you get jammed, just fly into a random direction for a minute or two and you're out of range of the jammer. Loitering near the jammer until you run out of fuel is just stupid. It would be hilarious if the RQ-170 actually did that.

@RainerP. In regards to the RQ-170: But for all I know, that is exactly what happend.
Anyways, programming a drone such that it accounts for every possibility is hard, takes forever and costs a lot of money. Keep in mind that every function you program has to be developed, tested and maintained. Often even no reduncandy is implemented in military drones, because it is cheaper to just write off the drone in the case of a mishap. You would be astounded by how many MQ-9 reapers are lost yearly without hostile influences...

Additionally I want to point out: Everything is obvious in hindsight :)

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

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Where is the CG on the Lockheed Model 10-A Electra?
I'd like to know where the centre of gravity on the Lockheed 10-A Electra is located (I am unable to find this information anywhere online). An image would also be very helpful!

And that's your answer...  (CG is normally around 1/4 of the chord length back from the leading edge BTW.)

As @757toga says, “It varies”  But here is an Electra flight manual with a Center of Gravity chart which a pilot would use to determine the CG for a given flight.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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Trouble with flight school and the change in flight instructors
How do I easily adjust to the instrucuting processess of a new flight instructor, So I can fininsh flight school early enough, I have had 3 flight instructors in the past 2 months and it has stalled my progress.

In what way are you having trouble or not adjusting?

Can you be more specific on what the actual problem is? While methods may vary a bit, your flight school should have a pretty much standardized curriculum, to which the instructors must adhere.

Understandable, every flight instructor has their own whacko view of flight dynamics, so it can be hard to switch once you have have figured out how to placate the one that you are working with.  I had an Embry-Riddle graduate flight instructor tell me that the plane would lose more altitude in a stall when flying downwind vs upwind.

This is more a life coaching request than an answerable question. I'd recommend you take your concerns to the chief pilot. There are simply too many variables in play for internet randos to be able to truly help you out here...

I often have new instructors approach a trainnig exercise that has been half way completed using a different apprroach, making pre flight and post flight brief very strenious.

Some instructors can be overcontrolling (and some moderators too).
I would focus on the curriculum of what you need to learn at school to gain sufficient proficiency to graduate, and to be a safe pilot.
It is difficult to change instructor sometimes, as communication is so important to work together.
But being aware and expressing your needs and feeling the instructor is addressing them is critical.
If you get stuck with someone who goes up with the attitude like:  OK, I'm just getting hours, just do what I tell you, consider switching again.
Although it is very important, in the early stages of training, to listen, as one progresses and starts to feel like they want to fly the plane, then a good instructor knows how to pass the reins to you and let you (safely) do it.
Landing on your own, and soloing, are milestones that we all achieve as pilots in command.
What ever style you instructors have, eventually you take charge of the aircraft.
Having multiple instructors can give you an advantage as they may present different points of view, and getting used to different co-pilots can be an improvement in skills as well.


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In FSX's Learning Center, PP, Lesson 4 (Taught by Rod Machado), how does Rod calculate the figures, "24" and "48" seconds in the Downwind Leg section?
So in Microsoft Flight Simulator X: Steam Edition / Boxed Version, the Learning Center incorporates a section for Flight Lessons, taught by Rod Machado. In Lesson 4: The Traffic Pattern, (in the Private Pilot category) under The Downwind Leg heading, Paragraph 3, Rod states the following:

At 60 knots ground speed, the airplane covers one nautical mile in one minute. Therefore, you'd want to begin the downwind turn anywhere between 30 and 60 seconds after turning crosswind. Since your airplane is climbing at 75 knots (75 knots ground speed assumed), you'll want to begin the turn sooner, perhaps between 24 and 48 seconds after turning crosswind.

How are the values "24" and "48" calculated here? I'm having trouble understanding the mathematical relationship between the 60 knots ground speed covering one nautical mile and the addition of time. (60 knots covering one nm in one minute) He doesn't explain how he gets these values and as such I'm struggling to understand the mathematical formula he's using here.

Compare the distance covered at 60 knots in 30 and 60 seconds, with the distance covered at 75 knots in 24 and 48 seconds.  What do you find?  To make the unit conversion easier, 60 kts = 101 ft/second; 75 kts = 126.6 ft/second.

That a really wide downwind!

This is very simple math:
The ratio of times is the same as the ratio of the speeds.  You can try calculating this yourself first, and check below for one way to do it:
So the speeds are 60kts and 75 kts, and the times are 30s to 60 s and 24s to 48s.
Find ratio of speeds:

 60/75=0.8

Apply the ratio to times:

 30x0.8=24 and 60x0.8=48

The faster you go the more ground you cover in a unit of time. Hence less time is needed to cover same distance.


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Can people perform major modifications to an aircraft without an A/P license, as long as their company is certified per part 145?
If a company is part 145 certified wanted to do a major modification, can they do so without having A/Ps? Or does an A/P need to be present to verify/sign off? or no A/P's need to be present?
At that point who would do verifications (if the company was certified per 145)?

It seems painfully obvious to me that part of the 145 certification process would be ensuring and maintaining a requirement that only qualified mechanics perform the work.  (perhaps with allowances for supervised apprentices...)  Why do you think it would be another way?

What do you mean by "does an A/P need to be present to verify/sign off? If you meant that they need to be present the whole time during the modification/Repair then the answer is no. A mechanic without an A/P can do modification/repairs, but they need a certified mechanic to verify/sign off their work.

You can likely find all the answers in 14 CFR Part 145 - Repair Stations.
But to provide a somewhat qualified and summarized answer to your question:
Per Subpart D - Personnel:

The stations is required to have an Accountable Manager, responsible
to ensure an adequately trained and certificated staff is in place to
do the work.
Supervisory personnel directly in charge of all the work to be done
and which are required to be certificated as a mechanic or repairman
under Part 65. Individuals performing the work under that supervision
must be trained, but are not required to be certificated.
Properly trained inspection personnel.
A person certificated as a mechanic or repairman under Part 65
authorized to approve an article for return to service.
If they choose to use repairmen, they must certify that the each
person recommended to receive a repairman certificate, a) Is employed
by the station, and b) meets the eligibility of Part 65.101.

You might want to also review Part 65 Subpart D - Mechanics and Part 65 Subpart E - Repairmen to understand the difference.
As a note to the second bullet above; from § 145.3 Definition of Terms:

(c) Directly in charge means having the responsibility for the work
of a certificated repair station that performs maintenance, preventive
maintenance, alterations, or other functions affecting aircraft
airworthiness. A person directly in charge does not need to physically
observe and direct each worker constantly but must be available for
consultation on matters requiring instruction or decision from higher
authority.


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Why do air planes not have a "grating" in the front of the jet engines for safety?
This is what a typical jet engine looks like:

Considering the horrible dangers with birds getting in there and crashing planes, why doesn't it look like this?

Why don't they have a "grating" in front like that, which would prevent the bird from ever getting into the blades?
Of course it doesn't need to look exactly like I have quickly sketched it up above, but why isn't that general idea used?

Jets are designed to deal with smaller birds. See https://www.nytimes.com/2009/01/22/nyregion/22engines.html#:~:text=Special%20offer%3A%20Subscribe%20for%20%241,Flight%201549%2C%20can%20be%20larger.

Of greater hazard to jets during near-ground operations is ingestion of rocks, dirt, dropped tools and trash. many soviet-era jet fighters designed to take off from relatively unimproved airstrips contain ballistic separators which are designed to strip out foreign objects from the airflow entering the engine inlets. In the west, this function was furnished by FOD (foreign object detection )patrols whose job it was to sweep the runway with a line of people shoulder-to-shoulder who searched for and picked up anything that the engines might suck up upon takeoff from aircraft carriers.

Even a well-engineered grate reduces engine performance significantly more than people realize.  Jet engine compressors, which are really rapidly-rotating little wings, are incredibly sensitive to pressure differences—managing pressure ratios is what they do.  Stationary industrial turbines are often offered with safety cages,  But making the “cage” small enough to reduce airplane drag reduces engine performance by about 5% which equates to billions in fuel costs.  It was found to be far cheaper to improve safety processes instead.


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How to determine the required twist in the Bell Shaped Lift Distribution (BSLD)
I'm doing a research about flying wings and BSLD and I'm having trouble to find the required twist. I'm using the equation L = ( 1 – x^2 )^3/2 to find the local load (Bowers, 2016). But how can I relate this to the coefficient of lift or twist of the section?
Thanks!

There is no analytical way to determine the twist for this (or most) load distributions.  You must do it either numerically or through guess and check type approaches.
We often think about the lift distribution $l(y)=c_l q c$ by dividing by the dynamic pressure $q=0.5 \rho V^2$ and focusing on the product of the sectional lift coefficient and the chord $c_l c$.
For an elliptical lift distribution, the downwash distribution is uniform.  This means that all wing stations 'see' the same angle of attack.  Consequently, if we shape the chord elliptically, we can achieve an elliptical distribution with constant $c_l$.  Since the downwash (and local aoa) are uniform, this means an un-twisted elliptical chord wing will achieve an elliptical distribution.  This is the only easy case.
For any other load distribution, the downwash distribution will be non-uniform and the twist required to get the desired local lift coefficient will be unknown.  In fact, it depends on the influence of the rest of the wing -- a full 3d solution.
Fortunately, there are ways.  For example, this paper documents a numerical method that can be used to twist any wing to match any target lift distribution.  This method has been implemented in a Python script to work with OpenVSP and VSPAERO.


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limits of the cg envelope graph for a helicopter
I am looking for information or any explanation, I want to know how this envelope is graphed, the limits (underlined in the image) from where they are obtained and what do they tell me? I want to know step by step how a graph is made as it is, specifically for a Helicopter, any idea where I can start I really appreciate it.

The basis of CofG location for airplanes and helicopters is completely different. For airplanes, CofG location is equivalent to the pivot axis of a weathervane relative to the weathervane's aerodynamic center. Static stability is achieved by making sure the aft-most CofG is far enough ahead of the aerodynamic center to get a strong weathervaning tendency in flight, positive static stability (but not so far as to prevent the "weathervane" from being trimmed to operate at an angle off-axis to get the wings to make lift, which sets the forward limit).
A helicopter is more like a camping trailer suspended from a crane, the rotor disk being the crane, and the mast being the cable.  If the CofG of the trailer is perfectly centered under the attachment point, the trailer will sit level. When you move the CofG forward and aft of that, the trailer will sit with a tilt.
The tilt means the vertical axis of the body is not parallel to the rotor axis in a hover (in still air). This angle-off-parallel uses up some of the range of movement available to the rotor disc, through the swash plate and control linkage, relative to the fuselage, needed to maneuver and to maintain a hover in various conditions, and to maintain a desirable nose attitude in cruise flight.
So, the helicopter's CofG limits are those beyond which there is not enough articulation range for the rotor, relative to the fuselage, to ensure control in all operating conditions.
The range tends to get narrower as the weight goes up as inertial effects of the higher mass can create dynamic effects in flight that further restrict how much articulation room can be given up in having the CofG off centre.
For example, aft CofG may be limited as weight goes up due to the need to have sufficient control authority to maintain nose attitude at maximum speed with some remaining forward cyclic margin, the more of this margin being used up the heavier the machine is, giving the slope at the back of the curve. Forward CofG may be limited until some fuel is burned off when the machine is heavily loaded, if the tanks are aft of the rotor axis, resulting in the slope at the front of the curve.
For that particular chart, the curves would have been determined by the designer initially, and validated and adjusted during development and certification test flying, where the machine is loaded to its limits at various weights and they see what happens.

Behaviour of the helicopter with CoG change depends on the rotor hinge

Thank you very much for your answer. But then, is there a way that I can make this graph step by step or a description of what is done to trace each point.
I understand that the designer is in charge, but then how does he determine the limits? From what I understood, data is also taken from various calculations for different helicopter weights?

@Koyovis Yes, if the rotor is teetering it's a completely free pendulous connection. If articulated it's more of a semi rigid flexible connection, but the CofG limits are still related to disc articulation limits one way or another.

It will be done from initially from calculations that would estimate disc displacement off axis for various weight/CG configurations, then validated by experimental test flights.  How much rotor margin is required for a rotor system will depend the type of rotor head and other things.  Then you have external features to help. When you see tail rotors that are angled, it's being done to provide a vertical lift component that allows extension of the aft CofG limit by helping to hold the tail up. Horizontal stabilizers also add to the potential CofG range.

None of this answers why the CG chart is shaped as it is. This is a very typical helicopter CG chart. Why are the circled points significant? Why are there slants at the top


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How does the secondary surveillance radar calculate the aircraft velocity?
In radar surveillances systems we have PSR ( primary surveillance radar) and SSR ( secondary surveillance radar)
And they provide informations to be use it by air traffic controller, one of these information is velocity :
PSR system : the velocity is Doppler speed.
SSR : send out interrogation at 1030 MHz and reply by the target at 1090 MHz, so it is not Doppler frequency.
My questions how does the SSR Radar calculate the speed ?

Even primary radar can't determine velocity from just one return. The Doppler shift only gives you the velocity component that's toward or away from the antenna site. Any motion side-to-side can't be determined by Doppler shift. Hence, most civilian radars ignore Doppler shift completely.
Rather, both primary and secondary radar determine speed the same way: by analyzing the difference in position of multiple returns from the same target. If a particular target's position during this sweep is 2000 feet away from its position during the last sweep, it must be traveling 2000 feet per 8 seconds (most common radars rotate one turn every 8 seconds; if your radar is different, adjust accordingly), which works out to be (let's see... carry the two...) about 150 knots.

And, of course, by "carry the two", I really mean "type '2000 feet per 8 seconds to knots' into Google".

Interesting.  Is there any logic for determining velocity of an aircraft who is performing a DME arc around the station?

@wbeard52 For an aircraft that's turning, the calculated velocity will be less than its true velocity. But the difference is rarely big enough to be worth considering, since radar only gives you ground speed anyway, and most planes only worry about airspeed.

And if the aircraft is Mode S equipped with Enhanced Surveillance (most today are), the Mode S ground station can just request the velocity (speed/heading) via Mode S datalink. It's the same data as broadcast via ADS-B.


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Why do ATR aircraft have a dedicated power management selector for each engine?
ATR aircraft have a split power management selector in order to allow both engines to be commanded seperately.

 Source: Magnar Nordal
but what is the purpose of having this facility? do pilots ever move the selectors seperately?

Disclaimer: I am not an ATR-72 pilot and much of this is speculation.
The video you have linked to contains the instructor saying the phrase

This was a requirement during the certification of the [aircraft]. However, we never split the selectors; we always keep them together.

My best guess is this. The throttle levers essentially control desired torque; in any situation in which thrust is degraded or asymmetric I am essentially certain that a combination of that and the condition lever is sufficient to manage a condition of asymmetric thurst. These two levers represent, approximately, the angle of the blades of the turboprop and the power generated by the engine.
However, the relationship between developed power, fuel flow rate, and thurst, is complicated and a function of altitude. The PWR MGT command essentially is an instruction to the engine's FADEC (engine control computers) about pilot intent and what phase of flight the aircraft is in. For the benefit of those reading this who have not seen the Youtube video, these positions are 'Take off' (TO), 'Maximum Continuous' (MCT), 'Climb' (CLB) and 'Cruise' (CRZ).
It is difficult to know these independently from measurements of the state of the aircraft alone: are you at FL150 and climbing because you are in the climb phase after take-off (so should set it to CLB), or are you at FL150, climbing 1000 feet following a chat to ATC, and about to begin your descent in two minutes – at which point CRZ is appropriate? The difference might seem minor but would affect the thermal management of the engine, for example, particularly over a long period of time.
I cannot easily find reference to PWR MGT being split in the flight crew operating manual (and I cannot find a copy of the airplane flight manual); its manipulation is routinely mentioned as part of normal operations for phases of flight such as take off (unsurprisingly). It is also mentioned in an incident report about a loss of control incident during an escape from severe icing conditions in Norway, in which the crew commanded maximum continuous (PWR MGT to MCT) in order to increase the prop's rotational speed and try to detach ice from prop blades, and decreased desired airspeed.
This all being said, I imagine it was the intent of the FAA that the ability to split the controls here is to ensure that any situation involving asymmetric thrust could be managed appropriately, whatever the condition of the engine, and despite the aircraft having a separate condition lever and power lever for each engine. I also imagine that it is possible to manage every situation with those two levers, which is why the instructor says the comment he does. However, when originally designed in the early 1980s, no aircraft was completely fly by wire (the Airbus A320 started that trend, in 1988) and it is likely that the degree of scepticism about the reliability of FADECs was both substantially higher and likely justified (the first commercial FADEC was on the Pratt & Whitney PW4000, which was certified in October 1986 -- the ATR-72 programme was announced in January 1986). The ability to make independent inputs to both engines provides another route in principle to deal with situations such as a runaway engine, or an engine whose performance degrades above a certain power level, and could therefore conceivably been of use in non-normal situations.
However, it would also be another thing to remember to adjust on a period of high pilot workload. The incident report I linked to above notes that

Descending to FL150, the crew reduced PWR MGT to CRZ. Engine power was
not increased again after the aircraft levelled off at FL150. That may
have contributed to the speed dropping more rapidly. According to the
severe icing checklist, PWR MGT must be in MCT (1.6.11.3).

This is an example of what I am talking about. A normal action appropriate in normal flight (switch PWR MGT to CRZ) is inappropriate in a non-normal situation in which PWR MGT should have remained at MCT. The pilots, being human, in this instance made a mistake and both omitted this memory item when concentrating on the activation of the stick shaker and later stick pusher. If it is possible to manage any conceivable asymmetric thrust situation with the condition and power levers alone, then this is clearly the preferred approach -- and that is why I think the levers are 'never split' as said by the instructor above.


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Is the aircraft in this image real?

(Image source)
At first, it appeared to be a freighter conversion of an Airbus A320 (known as the A320P2F). But upon having a closer look at the engine, it looks like a PW1100G, which would imply that this is an Airbus A320neo. However, to the best of my knowledge, Airbus never made a freighter variant of the A320neo.
Additionally, the winglets are different and remind me of the Boeing 737 MAX's winglets; I've never seen these winglets on an A320 before.
So is the aircraft in this image real? If yes, where can I find more information regarding this particular model?

I am 100% sure this is fake.
I found the same picture on Twitter in a tweet from April 28,2020 , but in higher resolution:

You can see the registration on the aircraft clearly: D-AVXA. This registration is an Airbus Test registration, used by Airbus in Hamburg. Below the registration, you can read the Manufacturer Serial Number (MSN) 8557.
The aircraft with MSN 8557 is a A321 which was given Test registration D-AVXF during production and testing. It was produced for Indigo as passenger version and delivered with registration VT-IUA. The last two letters (UA) of that registration can be seen on the nose gear doors of the freighter aircraft.
Clearly, something does not add up here, and that is only the first indication.
Then, I found a very similar picture of aircraft MSN 8557 in passenger configuration on planespotters.net. It also has the letters UA on the nose gear doors. Note almost the same position on Hamburg Finkenwerder airport (Airbus Assembly site), including the same shades of leaves in the trees (clearly an autumn setting, which matches the date on the planespotters picuture 16 November 2018).
The picture with the split scimitar winglets is dated in April, which does not match the background.
Zooming in on the lower part of the split scimitar winglet, it looks suspicious monochrome.
Also note that the Indian registration in front of the Indian flag is part of the paint job, and during testing this is covered with a white sticker. The sticker would overlap the Indigo CarGo Truck logo, but here the logo overlaps the sticker. The outline of the original sticker is still vagely visible in the edited photo.
Finally: the Twitter profile of the Twitter post says in the bio:

Skilled to Transform Your Wild Aeronautical Imaginations Into Digital Reality

That really gives it away.

Brilliant!!! -Niels

Thank you so much for such an amazing answer!

Not sufficiently skilled to give the truck round tyres: that bit looks like it was done with MS Paint.

Another giveaway are the 737 MAX winglets.

@MarkMorganLloyd to some degree, the tyres should not be round, due to the fuselage itself not being straight ... however this is the smallest of the issues, there are other paint-like features :)

I wouldn't expect the pixels to be /quite/ that blocky...

The question has already been thoroughly answered by DeltaLima, I'll just add that the original, unaltered, photo by Lars Hentschel can be found here:
https://www.airliners.net/photo/IndiGo/Airbus-A321-271NX/5285531

Great find! Welcome to aviation.stackexchange.com


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What is the purpose of MCP/FCU and FMS selecte altitude?
in Radar Mode S Asterix message, the  Mode S Enhanced Surveillance (EHS) provides

FMS Selected Altitude.
MCP/FCU Selected Altitude.

What is their purpose?
What is the differnte between FMS and MCP/FCU ?

Both are altitude inputs for the autopilot of the aircraft.
The FMS Selected Altitude is the Altitude selected by the Flight Management System (FMS). The FMS hold the flight plan as well as Standard Instrument Ddeparture (SID) and Standard Arrival Routes (STAR) procedures. These can be used to provide a Selected Altitude directly to the autopilot when the FMS manages the vertical profile of the aircraft (VNAV mode).
Another way to provide the autopilot with a refence altitude is to put it manually into the Mode Control Panel (Boeing parlance) / Flight Control Unit (Airbus parlance).
On some aircraft, the FMS can manage the vertical path in VNAV but the pilot can put an Altitude Intervention through the MCP.
In addition to the altitude fields, also the autopilot status and the current target altitude source is trasmitted in Mode-S.
The purpose of having these fields is that it allows the Air Traffic Control System to do checks (automated or manual by an air traffic controller), confirming that a descent/climb has started to the cleared altitude, that a level off is going to happen at a certain altitude etc.


text:
Why the ADS-B out doesn't broadcast the True air speed in its ES message?
The ASTERIX (version 2.1) items of ADS-B GS cat 21 contain True air speed but when we check the Asterix data that we recieved by GS we did not find the True air speed value, why ?

If you scroll down in the document to the description of true airspeed, it says "This item is optional."
Many aircraft do not have the capability to measure their true airspeed, and it makes no sense to transmit anything if the value is unknown.

Thanks for your reply.but isn't the optional mean it is optional to decoding this items?.also the whole aircraft that I checked transmitted the true Air speed via EHS register ( Radar interrogation)

@Uniform No, it says that under "encoding rules," not "decoding rules." An aircraft does not need to include true airspeed in an ADS-B message. Even if it is available, an aircraft is not required to transmit it.


text:
What is this Jeppesen approach plate symbol?

What is this “VVV” symbol circles on the plan view?

I can't find a reference for it in the symbology reference, but I'm pretty sure it indicates a route segment that can't be shown to scale.
The plan area is to scale, but the 5nm hold can't be shown entirely within the  chart.  So the hold racetrack is shrunk to fit and the route compression symbol indicates that it's no longer shown exactly.
The FAA charts have a similar symbol.

The FAA symbol is clearer and the more common version (at least, in general graphing; I’m not familiar with aeronautics). You’d also think they should include that symbol on both sides of the oval, since if you go out 5 nm you have to come back from 5 nm out and that’s also not drawn to scale. It’d help figure out from context what the symbol means, too—I rejected that interpretation because the other leg didn’t have it.


text:
Wording to call an aircraft from the ground
As a student pilot I've learned how to call "traffic", ATC, and other aircraft, when necessary. However, imagine I have a portable radio on the ground and want to talk to my friend who's flying nearby in uncontrolled airspace.

When I call him on the correct frequency, should I assume I'd address him tail number initially? (Though I've heard my instructors radio each other by name.)
How would I identify myself? Is there a "proper" way to do this? In uncontrolled space on unicom, does it matter?

You mention "uncontrolled" airspace a couple times. Are you certain that your friend flying lower than 1200' AGL? Because if they're higher than that they are in fact in controlled airspace.

@randomhead, well, of course that is location dependent...

@Michael given that OP used the "usa" tag, not really. Not in the Lower 48 and not even as much these days in Alaska.

@randomhead, but there is... in some locations in the USA... class G above 1200' AGL, yes?  And for purposes of the question, does it matter?  (The radio signal doesn't recognize airspace categories...)

In two remaining very remote and (relatively) very small locations, yes. The exceptions that prove the rule.

OK, but exceptions are exactly that, exceptions.  They don't prove a rule, they disprove it.

@MichaelHall The phrase "the exception that proves the rule" means that the existence of an exception proves that the rule exists, even if it's unwritten. The fact that there are only two remote areas that you can point to that don't fit the generalization can stand as proof that the generalization is accurate.

@DarthPseudonym, I agree with you, but the comment that started this was an absolute, not a generalization...

In short
When pilots are not operating aircraft, and are not performing another aviation-related duty, they are part of the public, and are not allowed to chat with aircraft crews using the dedicated aviation frequency bands.
If they perform an aviation duty requiring a radio station, a license has been issued for this station, with the corresponding callsign. From FCC:

You may only use your hand-held aviation VHF radio from your aircraft, or under the authority of an FCC ground station authorization.

Ground station authorizations are usually only issued to aviation service organizations located on airports, businesses engaged in pilot training, aircraft manufacturers, or persons engaged in chase activities related to soaring and ballooning.

Details
FCC regulates the use of the radio spectrum. The general rule is, a few exceptions apart, frequencies are allocated to services, and services cannot be used without a specific FCC authorization (license) for:

The transmitter, this is managed by the manufacturer.

The station, to deliver or use the service. This is the license we usually talk about.

Possibly the operator, e.g. boat radio operators or ham operators. This is usually part of a certificate recognizing knowledge and skills in a broader domain.

From the previous page:

Wherever aviation services are provided in U.S. territory, both FAA and FCC requirements must be met by anyone who uses aviation radio.

The law: 47 CFR
In the domain of aeronautical transmissions, frequency bands are allocated to navaids, ATC, voice and data communications, etc. In the US, everyone is allowed to listen to radio exchanges, even private, when the channel is not encrypted. However it is forbidden to transmit without the appropriate license.
47 CFR §87.18:

Except as noted in paragraph (b) of this section, stations in the aviation service must be licensed by the FCC either individually or by fleet.

Transmitting on dedicated aviation bands is allowed from properly licensed stations, operated by authorized users. E.g. a license is issued for the use of an aircraft transmitter by crews, for the use of a ground station by ATC operators, for a FBO station, for the supporting crew of a free balloon, etc. The license is issued with a callsign allocated to the station.
Such licenses are not delivered to a person, but to the station. From $87.18:

An aircraft station is licensed by rule and does not need an individual license issued by the FCC [...]

The stations must be used to provide one of the services described in Part 87, this is the meaning of "licensed by rule": The station stops to be allowed to operate as soon as it stops to be compliant with the technical specifications of the service or is not used for the service.
Regulations are similar across the world, because FCC and FAA work with other country counterparts within ITU and ICAO UN agencies. The international principles that need to be implemented in State member regulations are agreed within these agencies.
Communications on Unicom
A pilot on the ground is not allowed to use a transmitter to chat with flying pilots on the aeronautical bands, including on Unicom, where communications have specific forms and purposes. From the page previously linked:

Aeronautical advisory stations, also called Unicom stations, are land stations used for advising pilots of private aircraft about local airport conditions.

$87.18:

Even though an individual license is not required, an aircraft station licensed by rule must be operated in accordance with all applicable operating requirements, procedures, and technical specifications found in this part.

§87.213:

Unicom transmissions must be limited to the necessities of safe and expeditious operation of aircraft such as condition of runways, types of fuel available, wind conditions, weather information, dispatching, or other necessary information.

Not for chatting with pilots.
In practical
That said, in practical the FCC is not able to enforce the corresponding regulation and transceivers are freely available.
Still if an unlawful transmission disrupts or interferes with legitimate services or causes an incident, FCC and/or FAA may investigate and take the author to a court.
Bands available to the general public for private/business use
Services/frequencies open to the general public, without constraints on the content of the message include:

Citizen Band Radio Service (CB), HF, no license required.
Family Radio Service (FRS), UHF, no license required
Multi Use Radio Service (MURS), VHF, no license required
General Mobile Radio Service (GMRS), UHF, license required

There are other services, like the radioamateur service, open to the general public, but the license is granted after passing an exam and the content of the messages is (at least on the paper) constrained to technical topics related to radio experiments.

To expand on mins answer concerning amateur radio, in the USA and in most other countries, it is relatively easy to get a amateur radio or HAM radio license. In the USA a technician license lets you use VHF and UHF hand held radios. you only have to pass a fairly simple 35 question test. Many HAM radio clubs hold testing sessions, in person or even online, and some will hold classes on the material. There are numerous training apps, websites, and books.
With books, make sure they are relatively recent, the pool of questions on the tests get updated every 5 years. Our club had a mother and son fail the test, they had studies using a 30 year book from the local library. We showed them an app for their phone they can use instead.
These are not the aviation radios installed in the airplane. They operate in the same band, but on different frequencies.
Many pilots are HAM radio operators and do use hand helds when they fly to talk to other HAMS on the ground.
You cannot install a mobile HAM radio on your plane. There are regulations on what equipment can be installed. Hand helds are OK.
CB, FRS, GMRS, and MURS can also be used and each has its own licensing requirements. Some like FRS are very restricted in power and you might have issues with making contact over any distance.

I'm aware it's a usual practice nowadays, but using the amateur service like the CB service or the other services mentioned in the answer is not the purpose of the amateur service (research, testing and disaster communication), and is indeed prohibited as: "Communications, on a regular basis, which could reasonably be furnished alternatively through other radio services." (97.113).

Ham radio certainly is used for emergency communications and there definitely are people doing  a great deal of testing and research, but most HAM radio operators are simply talking to each other. There are definitely limitations, I cannot broadcast music for example. There are limitations on frequencies and power, but two people talking to each other, or even a group of people talking to each other, is definitely legal.


text:
Sense of parking names at GCTS
What is the reason behind the stand names in GCTS parking/docking chart? I can't find any. There are lots of airports with strange patterns but this one is plainly chaotic evil.
https://aip.enaire.es/AIP/contenido_AIP/AD/AD2/GCTS/LE_AD_2_GCTS_PDC_1_en.pdf

The stands on the airport diagram:

The table below the diagram:

From this table, the common characteristics by stand letter:

G: 400Hz AC is available
AG: anchor point available
J: compatible with A333
H: compatible with B763
R: some incompatibility with another stand occupancy
E: Nothing special.

The lesson I need to learn here is to always read further, even when I think I already have read enough. Thanks!

@Arget: That happens to me too, more often than I like.


text:
What are the advantages and disadvantages of a low stall angle?
One advantage I can think of is when we increase the wing's angle of incidence to decrease fuselage drag and increase takeoff lift, but would higher stalling angles always be better in terms of flight safety? And how low could a stalling angle be in terms of safety?

Increase the wings angle of incidence to decrease fuselage drag.

That's exactly how the B-52 bomber was designed and built in the 1950s.
Other very important factors are weight and airspeed in determining angle of attack necessary for flight.
What designers strive for is optimal wing area and angle of attack at cruise speed within a weight range to minimize fuel consumption.
This is why airliners are excellent gliders, with V best glide near their cruising (indicated) airspeed.
One would also sensibly limit weight so that Angle of Attack does not come too close to the stall limit. However, with adequate power, one can increase the amount of lift being produced by the square of the increase in airspeed.
This is a far safer approach than using increase in AoA to increase lift, as drag increases exponentially as AoA approaches stall.
Finally, "drooping" the nose does not affect wing AoA, but for lighter models (stored in windy places) it is good to be able to get 0 or negative AoA on the wing.  Tricycle gear can help this, with the nose wheel a little lower. Make sure you have enough elevator authority (in design) to rotate at takeoff speed.

The B-52's wing has a relatively high angle of incidence because the B-52 cannot rotate at takeoff due to the very aft landing gear, not to decrease fuselage drag.

Don't forget pilot vision: The F8U Crusader even jacked the wing up so the fuselage angle during approach and landing was low so the pilot could better see where the plane is going.

@sophit well, after the gear is up then it really gets to be an $airplane$.  Lowering fuselage drag helps.  I wonder, instead of swinging the wing back and forth, perhaps an adjustable incidence ...

Yes, after the gear is up then it really gets to be an airplane but with a wing with an incidence so high that it has to fly with the nose slightly pointing downward. This doesn't help in reducing drag.

@jkztd bingo! And slats can be used to lower wing AoA.  Fuselage drag bears study and should be minimized.  Variable wing incidence may help.


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Can I identify wingtip stall by looking at the lift distribution?
I'm projecting a wing in XFLR5. The lift at the tip of the wing looks like it's dropping very quickly close to the stall angle, so I think there may be wingtip stall, however, is there any way I can confirm that? I thought about comparing the lift coefficient of the wing with the maximum lift coefficient of the airfoil, and If the local lift coefficient is bigger than the maximum airfoil lift coefficient, I can conclude that region is stalling. Is this thought process correct, or is there any other analysis I could make? If so, which Reynolds number should I use to acquire the airfoil lift coefficient, should I just consider the aircraft speed in the formula?
Here's the image of the lift distribution, if it's useful having a look at it:

Is it possible to plot also the distribution of drag and pitching moment?

@sophit Sure. Here's the drag and the pitching moment distributions. If the wingtip is stalled then the drag should have a significant increase on the tip, which isn't the case here? Although there is a very small increase, does this say anything? And for the pitching moment, we should look for a high increase at the tip as well? Comparing with other wings, it seems the pitching moment difference between the root and the tip is higher in this wing, does this indicate anything?

According to the documentation, xflr5 is a simple panels method solver i.e. it definitely cannot predict stall or any other viscous phenomenon. Maybe what you can do is get the plot of the lift coefficient vs. alpha for each airfoil and check if at the local Cl that you get from xflr5 the relevant airfoil is stalled or not.

Yes, as plotted the wing will stall at the tips first. Consider to reduce the taper ratio (i.e., make the tip chord larger).
To predict stall, angle of attack, the rate of angle attack increase, wing sweep, wing aspect ratio, Mach number and Reynolds number all have an influence. For a single wing Mach number and rate of increase are the same, but taper will reduce the Reynolds number at the tips, so stall will occur at a slightly lower angle of attack there than at mid-chord. Therefore, your local lift coefficient should have at least a stall margin of 20% at the tip when it reaches its 2D stall value at mid-span.
Increase this margin if you have ailerons and especially if there are flaps at mid-span. Slotted flaps in the center section should be combined with slats on the outer wing to avoid tip stall.
Giving the outer wing panels more dihedral also helps to delay tip stall since the angle of attack will increase there only with the ratio of the cosines of the inner and outer wing panels.

Just to verify if I understood correctly, the local stall coefficient at the tip must be at least 20% smaller than the maximum 2D lift coefficient? And what about applying twist to the wing, would that help the tip stall problem as well?

@LuanArita Not stall coefficient, but lift coefficient. Yes, twist will also help. If you hit the maximum lift coefficient near the center while the outer wing is 20% or more below its maximum lift coefficient, stall should be benign. Make sure to pick an airfoil with gentle stall characteristics. 5-digit NACA airfoils are NOT recommended.

As far as I understand, XFLR5 is not able to predict wing stall.  The lift distribution must return to zero at the wing tips, it will often do so fairly sharply.
That said, this sort of analysis is often used as a crude predictor of CLmax and also the location of the wing where stall occurs.  However, instead of plotting the lift distribution (cl*c), you should plot the distribution of the lift coefficient (cl).
In this case, because c tapers at the tips, where cl*c is relatively constant, cl will increase even more sharply.  I.e. overall the cl distribution will have peaks in the outboard regions.
You can compare the sectional cl value with an estimate of the 2D clmax you expect fo this airfoil (and Reynolds number).  Lets say you expect 2D clmax to be about 1.4 (and the whole wing uses that same airfoil), then you can repeat the analysis for a range of alpha, plotting the cl distribution each time, and when the first section reaches 1.4, you might expect the start of stall.
This also tells you whether a wing will stall first inboard or outboard -- perhaps where the roll control is (bad news).
This general approach is not perfect, but it isn't terrible either.  While it has no rigorous basis, it has been used in conceptual design for a very long time.  It will work better for a subsonic, un-swept wing like this one.


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How to determine thrust and power requirements from take-off ground roll distance?
I'm working on a project which requires me to determine the power requirements of a propulsion system from a specific take-off distance. Starting with this equation:
$$
    s_g\approx \frac{1.21(W/S)}{g\rho (Cl)_{max}(T/W)}
$$
I rearranged to get
$$
T=\frac{1.21(W^2/S)}{g\rho Cl_{max}s_g}
$$
My question is: Is this the total thrust at takeoff, and if so, is this (following equation) a valid method of determining the power requirements?
$$
P_{req}=\frac{TV_{lo}}{\eta _{prop}}
$$
Thanks in advance!

How did you get that equation?

@sophit I took it from the textbook "Aircraft Performance and Design" by John D. Anderson

There's no explanation about how to use it in the book?

@sophit returning back to the textbook it mentions that T/W in the original equation is evaluated at a velocity of 0.7Vlo (average T/W using average velocity), does this mean its not valid to use the actual lift-off speed? I would like to figure out the absolute max thrust and power required throughout the takeoff and climb

"quick & dirty" equations like that one normally refer to "nominal" conditions: T is the max thrust available @ all engines operative, W it max takeoff weight and S the nominal wing surface. You plug everything inside and you get the field length or the needed T/W to stay under a certain length. This answer should also help you.

@sophit perfect! thank you very much

You're welcome


text:
Are humming birds more like planes or helicopters?
Hummingbirds fly, helicopters fly, planes fly. They all have tails, however, planes generate lift with thrust and wing shape, helicopters generate lift via their spinning blades birds use both similar principles to planes and helicopters. Hummingbirds flap their wings generating lift and thrust like a helicopter and they can stay in place like one, and they glide easily. Their wing shape also helps them in flight. So, what is closer?

Welcome, but I don't think this question is a good fit here.  For one, birds are really nothing like either, so there is no real answer to the question.  All you will get is opinions and chat.

By a flight characteristics point of view I'd say a helicopter (an airplane cannot hover). By an aerodynamic point of view I'd say a cyclogyro.

A hummingbird wing is more like an insect wing than either of your examples because it exhibits very large torsional flexure while being flapped and it generates lift on both strokes (forward and aft).

+1 This is the best answer I've ever seen that doesn't answer the OP's exact question.


text:
Field and Alternator Breakers
I under stand how the field and output breakers work within the electrical system. Monitoring the voltage output, I have a constant 13.5 volts at idle. During run up, the voltage will climb to ~13.8-14 volts while showing a constant +40 on the ammeter. Approaching 2000 rpm, the output and field will both pop at the same time. The battery has a resting voltage of 12 volts so I don't suspect a bad cell. I feel I should be looking for a short, but dont understand why both would pop at the same time unless the alternator itself is shorting.

If your output breaker is tripping, that is serious. It takes a load of something like 60 amps to trip. Stop messing around and get it repaired by a professional.

I 100% agree with @MikeSowsun, but if I were to take an educated guess, your voltage regulator has given up the ghost. Regardless, this is not an issue for self-troubleshooting unless you're already an A&P (which I suspect by the wording of the question that you are not.)


text:
What is the engine power and brake configurations when starting takeoff?
When taxiing from the gate to the runway, there is a near-constant engine sound and a near-constant speed (at least from my perspective as a passenger). Once we get to the runway and align for takeoff, the engine sound reduces to near-zero and we are not moving.
Now, while starting to takeoff, I have often noticed that the engine sound increases significantly (much more than the sound levels during taxi), but the plane does not appear to move for a few seconds. And then all of a sudden, we start moving. I had assumed my mind was simply not registering the movement, but I flew on a A350 (taking off from ORY runway 07) a few days ago with a live tail camera feed and it was quite obvious during those few seconds that we really were not moving.
I found this video where the pilot of an A350 appears to hold on to the brakes while increasing engine power and then releasing it to start takeoff.
Why is this done? My guess is that the pilot does not want to lose those few tens of meters accelerating with lower engine power, but I can't imagine that would really be a problem. In fact, wouldn't this lead to more wear and tear of the brakes? And the runway length is 3320m while an A350 needs 2600m, giving it what appears to be plenty of room.

FYI, holding the brakes doesn't cause wear and tear.  They only wear down if applied while the wheels are rotating.

This is for equalizing the engine N1 RPM if there is any discrepancy between them.
So, a plane's jet engine "spools up" before takeoff and holds at a specific thrust level to ensure all engines reach a consistent power level simultaneously, preventing uneven thrust during acceleration which could lead to loss of control during takeoff.

Why pilots should set thrust in two steps for takeoff?
The stabilization step ensures that all engines reach a rotation speed value from
where the increase of engine thrust will be almost identical to each other (as shown in figure below).

The N1/EPR/THR stabilization value is defined during flight test campaign
for every engine type with collaboration from engine manufacturers.

Example on how it's done
Airbus A310:

Slightly advance throttles and monitor spool-up
until both engine are above idle (approx 40% N1)
or
Slightly advance throttles and monitor spool-up
until both engine are aligned and stabilized
between 1.05 and 1.10 EPR with no more than
0.002 EPR difference between both engines.

About your question about takeoff conditions... during takeoff usually it is never so on the edge that a pilot would want to not lose a few meters of the runway accelerating at a lower thrust.
In fact, most of the takeoffs are done with a lowered than maximum thrust which takes advantage of the fact that most runways are very long and planes have more than enough thrust to takeoff without using the full runway or the full thrust.
This is called Flex Temp in Airbus and Reduced take-off thrust in Boeing.

EPR = Engine Pressure Ratio is the ratio of the turbine exit pressure to the engine intake pressure
N1 = Speed of the low pressure spool and serves as the primary power setting, and N2 is the speed of the high pressure spool


text:
Hexagonal Texture on RB211 Turbofan Intake
I was at the science museum in London, and was looking at the Rolls Royce RB 211-22B. This particular engine was, I believe, the sixth one ever produced. The surface texture of the fan inlet looks like a half-complete honeycomb, composed of grey and black dots. Does anyone know what the function of these dots are, and, why they seems to be arranged in this semi-random hexagonal structure? My guess is something to do with thickening the boundary layer before it hits the fan tips, but I am especially curious as to why they chose such a specific structure.

Probably an acoustic liner, as explained here.

I believe this is an acoustic liner.  The holes on the surface are on a regular grid.  Underneath, there is a honeycomb layer.  That honeycomb is also uniform in size.  The alignment / misalignment of the holes and the honeycomb give rise to the semi-random looking pattern.

Thank you - you're right! I wonder if the misalignment between the holes and the honeycomb is deliberate (as opposed to each hole being in the centre of a honeycomb). Perhaps the reason for the misalignment is so that multiple acoustic harmonics can be dampened?

I know vanishingly little about acoustic liners.  I know that the size of the hole and the size of the volume behind the hole are very important parameters.  I have no idea about the effect of multiple holes vs. a single hole per chamber.

Maybe to avoid any resonation in the flow pattern?


text:
Can you be a pilot with level 1 ASD?
I'm not 100% sure, but I think I have level 1 autism spectrum disorder (formerly called "high-functioning autism" or "Asperger's syndrome"). I have never been diagnosed, and I have never had an appointment with a psychiartist or psychologist for any reason. I want to get a private pilot license at some point, and possibly CPL.
Does possibly having ASD affect this? Will I have to tell the AME that I suspect that I am autistic? Would being diagnosed with ASD be a problem?

I sometimes get the sense that young people are searching for some condition to latch onto to explain their perceived personality flaws. If you're not 100% sure, and nobody has ever brought it up or made any kind of assessment, and you live a more or less normal life, why are you even bothering with this? Just live and go on with whatever it is you want to do. If you have serious emotional issues that could impact your ability to fly airplanes, it will come out during training.

It isn't just young people, or those supposedly afflicted - grad students, PHD candidates, clinicians, all need to "publish or perish" in order to further their professional progress.  They are the ones forever performing new studies, and creating new categories of illness and disorder for people to fret over.

Just a suggestion, but try shifting the question away from the named thing, (ASD) and ask instead about the specific ways it might manifest itself and whether they might disqualify you.  Because the name is nothing more than a label, an administrative fiction, while the symptoms themselves are the things that are real. What is it about you specifically that makes you doubt your ability to perform as a pilot?

ASD is not specifically disqualifying, and is discussed in a Federal Air Surgeon's Medical Bulletin here. While it isn't specifically disqualifying, you can expect it to take a large amount of time and money to convince the FAA that you are fit to fly.
If you are diagnosed with ASD, you will have to disclose it to your AME. This will very likely cause your case to be deferred to the AMCD, who will consider the totality of your case. This may result in a relatively quick (less than 1 year) approval. It may also result in a long, expensive process that eventually ends in denial. Or anywhere in between.
Note the airman discussed in the medical bulletin was issued a medical, but his symptoms were light and did not require treatment. If you function well and don't see a reason to see a medical professional, I see no reason you would legally be required to disclose a medical condition that, to the best of your knowledge, you do not have and have never been diagnosed with. And I would advise in general that you do not disclose anything to your AME that you are not required to.
If you are diagnosed or if you choose to disclose that you think you have ASD to your AME, definitely find an AME with experience dealing with cases like yours, and have a consultation before going in for your medical. The FAA will want documentation, and if you start the medical prematurely you will not have time to get these documents before your AME has to send your package to the FAA. This will slow down the process considerably.

Thank you! If I do have ASD, the symptoms don't significantly affect my daily life.

@user68439 The criteria in DSM-IV (the AMCD still uses this supposedly) used to diagnose Asperger's can be found here. In particular "The disturbance causes clinically significant impairment in social, occupational, or other important areas of functioning." So if it doesn't affect your daily life, you by definition don't have Asperger's, by my reading of the criteria.

Not every neurodivergence is a "disorder," and ASD level 1 is a disorder, identified by needing some support to make it through daily life. In DSM-V clinical symptoms are rated from level 0 to level 3. You can be neurodivergent and still fall within level 0, which is not a disorder and should not lead to a diagnosis of anything.

Autism isn't a disease, nor is it a disability in and of itself (even though allistic people's behavior towards autistic people can very well be disabling). It is just a natural variation in human neurology, just like some people are white and others are black.
As such, I wouldn't disclose autism to an AME at all, especially seeing the discrimination that we are so often a victim of.
The FAA doesn't even list autism as a disqualifying factor whatsoever, yet many AMEs ignore that and discriminate against autistic pilots by putting the undue burden of further psychological testing on them.

Isn't it the job of the AME to try to ensure that candidates are fit to fly? If there is a question, doesn't it need to be answered before a medical certificate is issued?

It sounds like the bigger issue is that pilot certification bodies need to determine what types of autism, if any, can impact one's ability to be a safe pilot. I have friends with autism that can possibly be safe pilots (I am not qualified to make that evaluation, so I simply use the word "possibly", as I would for most candidates), but I also have friends with autism for whom, unfortunately, the autism would very likely make them unsafe pilots (or vehicle drivers). Of course, the same is true for everything that can impact performance, including age, vision, alcohol/drug use, sleep, & stress.

"I wouldn't disclose autism to an AME" I think you may be required to disclose any medical diagnosis, but there is nothing I'm aware of that requires you to seek out a diagnosis for something you weakly suspect you just might have.


text:
Were contrails different in the 80s?
My parents insist they remember contrails being significantly shorter around 1980s.
From my research it seems this might be the result of several factors such as climate change, advancements in aviation technology (engine design, fuel types) or changes in common aviation procedures. Of course, this may also be bad memory.
Can that be factually true?

Related: How does engine exhaust temperature affect contrail formation and why? and How does contrail formation differ from turbofan to turbojet?

Thank you for the links. Are there any specific historical changes that are generally understood to have affected contrails?

Well, according to those answers and the referenced paper a turbofan, i.e. a more modern engine, generates more contrail than a turbojet, i.e. an older engine.

I remember contrails in the 1970s arcing across much of the sky.

There could be a few things:

They are not remembering contrails, they are remembering exhaust. Old jet engines, particularly the early turbo fans and low-bypass turbo jets, as well as early water injection systems were prone to be much smokier. Advancements in understanding of engines as well as high bypass turbo fans have mitigated this quite a bit. So what your parents may be remembering are actually exhaust trails.

Depending on how old your parents are, aircraft have cruised at various altitudes over the years as structural technology has allowed better pressurization, and jet technology has allowed higher cruise altitudes to take advantage of trade winds. Since contrails are based on ambient moisture content, a given aircraft's cruise altitude will affect their behavior and appearance due to various environmental factors.

Humidity matters; as the FAA notes there are both short lived and long lived contrails. So unless your parents have been observing standard aircraft routes over a fixed point, through all seasons, for years, varying conditions will change the formation and life of contrails as ambient humidity fluctuates.

After the initial formation of ice, a contrail evolves in one of two
ways, again depending on the surrounding atmosphere’s humidity. If the
humidity is low (below the conditions for ice condensation to occur),
the contrail will be short-lived. Newly formed ice particles will
quickly evaporate as exhaust gases are completely mixed into the
surrounding atmosphere. The resulting line-shaped contrail will extend
only a short distance behind the aircraft (See Figure 2).
If the humidity is high (greater than that needed for ice condensation
to occur), the contrail will be persistent. Newly formed ice particles
will continue to grow in size by taking water from the surrounding
atmosphere. The resulting line-shaped contrail extends for large
distances behind an aircraft (See Figures 2 and 3). Persistent
contrails can last for hours while growing to several kilometers in
width and 200 to 400 meters in height. Contrails spread because of air
turbulence created by the passage of aircraft, differences in wind
speed along the flight track, and possibly through effects of solar
heating.

I would read up on the linked brief from the FAA, which has a lot of interesting contrail info.
Case in point, it's pretty dry where I am today; here's a short contrail I just observed from my front porch:

(source: own photo)

Unrelated to the topic, but in #2 you mention trade winds, is there a reason you say this and not jet streams? I was under the impression that trade winds were relatively close to the surface of the earth as they were used by sailors for trade routes.

@Tanenthor I sail a lot as well as flying, must have been top of mind.

I strongly think it's the humidity thing. In my experience, contrails looked basically the same in the 80's as they do now, and they've always been quite variable, sometimes short, sometimes long, and sometimes absent entirely.

Years and locations may be relevant here.   I have distinct recollections of 1982 where El Nino weather patterns were severe world-wide.

In the eastern U.S., it was the warmest winter in 25 years.
source

And air temperatures have a direct relationship to condensation and dew point.

My parents are from Volgograd. As far as I can see basically no soviet climate data exists online, so I'm looking at NASA datasets to find out if there were any significant climate changes in that area

I'm wondering whether modern high-bypass jet engines are a factor. They create a core of exhaust gases surrounded by a swirl of unmodified atmosphere. Two benefits are increased fuel efficiency, and reduced noise (this latter because the exhaust noise is trapped by the swirling air).
Possibly, this swirl also confines the water in the core exhaust for longer, wgich would make for a longer con-trail in an atmosphere where the water is evaporating, or a denser one where water is condensing on combustion-generated particles.

That was my thinking. The bypass ratio of the CFM56 family used in B737s  of the 1980s was 6.0 or less, while it is 9 or more  for modern engines. This effectively reduces the water vapor contents by a factor of almost 2 and probably leads to lower exhaust temperatures. It would be surprising if the condensation patterns were the same, actually.

Comparing 1980 to 1990, satellite photos reveal a

significant decrease in contrail cloudiness over the [sic] western Europe and a significant increase over the [eastern] North Atlantic between March and July.  ... Causes, possible errors and consequences are discussed.

Volgograd is distant from both of those, and the compared time periods match, so your parents' report isn't implausible.  But unless they kept written records, their recollection is hard to reconcile with others' recollection, in Volgograd or elsewhere.

I live on East coast of the UK, right under a bunch of major airways that connect middle East, mainland Europe with North America.  On a clear day we can usually see several aircraft in the sky at any time.
Roughly 10-15 years ago we had never seen a persistent contrail over here.  I had seen photos of them from places like America.  We either had NO trail, or just a few seconds worth.
One day the sky was covered with trails.  It was the first time we ever saw them.  Facebook (which was new) had tens of pictures from people all over town, posting what none of us ever saw before.
As the days passed by, nobody ever mentioned it again, the trails remained every other day from that point onwards, and it soon turned into "normal" and now I think it's "always been that way" in many peoples mind.
The default AI Google responses simply don't apply here. We always had tons of air traffic, and the weather (besides that effect what trails hiding the sun bring) has not changed.  The high bypass engines which apparently cause worse trails, has been a thing for decades, so that doesn't explain it either.
Remember we went from none at all, never seen, to everywhere, every day.  My best guess, is that somehow the humidity up there is higher than it used to be, so the conditions to form the trails are ripe virtually every day.

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text:
Why does a wing's pitching moment distribution tend towards zero at the tip instead of towards cm0?
I am currently playing around with XFLR5 and am trying to wrap my head around the following:
For airfoils, my understanding that the pitching moment has a component that is independent of lift, which is cm0, or the pitching moment at zero lift.
In XFLR5, I loaded a NACA2412 airfoils and ran the analysis, yielding the following plots:

cm is pretty constant between -1<cl<1 (which I think indicates that the aerodynamic center is close to 25%c) and consequently, cm0 is roughly -0.05.
Now, when I apply this airfoil to a simple trapezoidal wing and run the analysis, I get the following lift and pitching moment distributions (at 0 and 10 degrees AOA):

The local lift coefficient varies between 0 and a little more than 1, so far so good. From the airfoil 2D data, I would now have expected to see a somewhat constant pitching moment distribution across the span (somewhat like a lookup funtion: local cl = 0.5, therefore local cm = cm.2D(cl=0.5)). However, the analysis shows this:

In the center of the wing, we're seeing what I'd expect. At the tip however, cm goes to zero, or rather seems to scale with the cl distribution, instead of being constant.
Why are we seeing this behaviour at the wing tip, which seems to be in contradiction with the 2D data?

In you last picture, which plot (line, line with circles) is which? And which one exactly would you expect?

You are thinking that a section at the tip of a wing will behave like a 2D airfoil.  This somewhat happens at the center line due to symmetry (and distance from the tips), but the opposite is true at the wing tips.
Lift is the integral of the local load across the span of a wing.
$L = 0.5\,\rho\,V^2\,\int_{-b/2}^{b/2} c\,c_l\,dy$
But that includes the local lift coefficient, which is an integral of pressure across the chord...
$c_l = \frac{1}{c} \int_0^c \left(C_{p,l}-C_{p,u}\right) dx$
Where $C_{p,l}$ and $C_{p,u}$ are the pressure coefficient on the upper and lower surface of a wing.
We can combined these two integrals together to think of Lift as an integral of the difference in pressure coefficient from the top to bottom of the wing over the entire wing area.
$L = 0.5\,\rho\,V^2\,\int_{-b/2}^{b/2} \int_0^c \left(C_{p,l}-C_{p,u}\right) dx\,dy$
This makes the most sense if you think of a wing as a thin camber surface (like from thin airfoil theory).  This is very much the case when you use VLM or LLT theory (two options in XFLR5), but is a little bit of an abstraction if you're using a thick-surface theory like a panel code or CFD.
In those cases, things are a little more complex, but it still works out pretty much the same.  For now, think of a wing like a 3D version of thin airfoil theory -- a thin camber surface plus a symmetrical thickness form plus a flat plate at an angle of attack.  Remember the thickness form does not contribute to lift or moment, so we're safe to ignore it here.
Shrink yourself down to that differential element $dx\,dy$.
If you were to walk a line at the wing tip, you would find that you are in a place where pressure and a surface doesn't work like normal.  One infinitesimal increment outboard (off of the wingtip) and you clearly have no surface to push on, so the air pressure does not contribute to the force/moment of the wing.  However, without a surface to push on, the lower and upper pressure coefficients are equal $C_{p,l}=C_{p,u}$.
One infinitesimal increment inboard (into the interior of the wing) and you clearly contribute to the integral like normal.  The surface of the wing can support the pressure difference that you integrate up to become a lift force.
However, when you are exactly at the wing tip, what happens to $(C_{p,l}-C_{p,u})$?  Is there a surface to support a pressure difference?  If there was a pressure difference on that edge, what would happen (wouldn't the air just escape around the edge?  To the point that there would be no pressure difference.
As it works out, aerodynamics requires that the upper and lower surface pressures are equal exactly at the wingtip.  Consequently, that exact line can not contribute to the forces and moments.
Although I've constructed this argument thinking about lift (not moment), the equation is the same -- except the integral includes a moment arm as a distance from the reference point (typically c/4) to the point of integration (x,y).  This term would look something like $(x-c/4)$ or maybe $(c/4-x)$ multiplied by the $(C_{p,l}-C_{p,u})$ term.  This contribution to pitching moment will still go to zero because $(C_{p,l}-C_{p,u})$ always goes to zero at the tip.
Check out the distribution of spanwise induced drag coefficient.  It too will go to zero at the tip.
So, like I started out -- flow at the centerline of a wing behaves very similar to a 2D wing.  However, at the tip, flow is so dominated by 3D effects (the presence of the tip) that it can't even support a pressure difference from top to bottom.

"This is a VLM code that represents the wing as an infinitely thin lifting surface" the wake is represented as a thin surface, not the wing nor the fuselage

Check this doc http://www.xflr5.tech/docs/Part%20II:%20The%20inviscid%20problem_rev1.1.pdf slides 22, 23.
It appears that XFLR5 has an option for a thick-surface panel formulation in addition to the thin-surface VLM and LLT formulations.  We do not know which the OP is using.
However, that does not matter for the rest of my answer.  Even at a thick wingtip, wings can not support a load and the upper/lower pressures must be equal.

Let's set the computer and the formulas aside for a moment and try to approach the issue with a mechanistic understanding of wingtip airflow, but first ...

what type of airfoil are you using???

The NACA 2412 has zero lift at -3 degrees AoA.  Slightly non symmetrical.  The Cm increases with AoA, but remains negative through stall, in agreement with your first 2 graphs.

why would Cm increase near the wing tips?

You have 2 plots, 0 and 10 degrees.  We see decrease in coefficient
of lift near the wing tips and an increase in Cm.

could this be real?

Because the wing tip vortex is expected to curl up and over the wing tip, striking the back of your trapezoidal wing, this is exactly what one might expect.

how do designers help this?

WINGLETS
Notice the wing tips of the seagull, tapering down to a point.
Try this type of wing tip on your XFLR5 and please let us know how your 0, 10 degree Cl/Cm plots turn out.

According to the user manual, XFLR5 lacks the simulation of viscous boundary layer; this implies that you should "limit the analysis to conditions of limited flow separation:

High Reynolds numbers
Low angles of attack
Low flap deflections"

Furthermore, "the wake is modelled as a straight extension of flat panels behind the wing’s trailing edge" but in reality "the wake takes the shape of the streamlines, and the wake panels should take the shape of the streamlines".
This implies that the primary viscous, 3D effects at the wing tips are completely lost.
Whatever you get outside these conditions is just random numbers.

Yes, but both at 0° and 10° AOA the sim should still be well within the linear part of the c_L-alpha-curve, so I don't expect any separated flow anywhere.


text:
Why does minimum pressure point move towards leading edge with aoa?
why is it that the minimum pressure peak over the top surface moves towards the front of the aerofoil with angle of attack increase?

The minimum pressure point -- the suction peak -- is the location of the highest velocity around the airfoil.
The flow must accelerate to get around a turn, so this ends up correlating with the curvature of the airfoil.
At zero alpha of a symmetrical airfoil, there is a streamline that hits the leading edge exactly -- this streamline comes to a stop and is called the stagnation streamline.  Every streamline above the stagnation streamline goes over the airfoil.  Every streamline below, goes under.
The streamline immediately above the stagnation streamline can be thought of as the streamline 'nearest' the airfoil all the way around.  It does the tightest turning.  When the airfoil is at zero degrees alpha, its path is somewhat mild as it accelerates up and around the airfoil.
When the alpha increases, the stagnation point (where the stagnation streamline meets the airfoil) moves to the bottom surface (below the leading edge).  The streamline just above now must flow up, around the leading edge, and then down the top surface of the airfoil.  It has to make a very sharp turn all the way around the leading edge.  This requires a lot of acceleration and leads to very high velocity -- a very strong suction peak.

Hi Rob, how does the flow accelerate over the leading edge itself please?

@MintyPythonn Somewhat sloppy description (exact description is with a set of differential equations) is that the air can't go through the wing, so it is deflected up. It then tries to continue straight by inertia as the wing curves away from it, but then there is less of it near the wing, so low pressure region forms and the pressure gradient accelerates the air towards the wing and aft. The flow has to obey the Bernoulli's principle that lower pressure means higher velocity, but note that that is an equivalence, not cause and effect and the pressure gradient is closer to being the cause.

Whomever drew those "pressure distribution" vectors doesn't know what he/she is doing.
Two points: 1. Sometimes we "normalize" pressure values by subtracting the absolute value, so we can represent lower than free stream pressure as suction, (which it is not). To accurately portray pressure vectors,  you need to display their absolute values. They should all point towards the wing surface.
2. Secondly, pressure is the result of aerodynamic force of air molecules impacting and rebounding off the wing. The force exerted on a wing from the result of the impact of those fluid particles on a surface is caused directly by the change in momemtum of the particle when it changes direction.
F=ma, or.
F=m(dV/dt) or.
F=d(mV)/dt i.e.,
F=dP/dt  (P is mV momentum)

Understanding this makes it much more intuitive, (qualitatively), why the pressure distribution diagram looks like it does, and why it changes as AOA changes. Molecules impacting a surface are no different in this regard than a baseball impacting a bat. The steeper the impact angle, and/or the higher the velocity, the higher the force will be.

Hi, sorry I still don’t get why the point of  minimum pressure moves towards the leading edge with aoa? I just found them online

@Minty Python, If the higher pressure occurs at the higher impact angles and velocities, then the higher the AOA, the higher impact angles (and the higher velocities at impact), occur in front of the aircraft, not at the rear.


text:
Weight of aircraft; is it mass or weight?
Books using the equation, Lift=Weight, to calculate stall velocity or lift coefficient for cruise condition or something like that.
Problem is that, weight of aircraft is written by unit of lb. or kg which are actually mass. This makes me confusing since the equation Lift=Weight is not valid for mismatch of dimension; lift is in N, weight is in kg.
Can anyone explain about this?

Weight is a force, and therefore should be measured in units of force. But, of course, humans have very little experience with environments that don't have 1g of gravity, and thus routinely conflate force and mass. This kind of thinking goes far beyond aviation; you can find it everywhere. For instance, spring-loaded bathroom scales measure force but will almost always display a result in kg (in metric countries). It's sloppy, but it works, and so there's little incentive to change it.
The pound is both a unit of mass and a unit of force. A one-pound mass will produce one pound of force in standard Earth gravity. If for some reason you need to specify which one you're talking about, you can use the terms "pound-mass" or "pound-force" to differentiate*.
There's less of an excuse for this in the metric system, which has separate units for force and mass, but that doesn't stop it from happening anyway. If need be (such as when reading a manual that uses kg as a unit of force), you have to think of "kg" as "newtons divided by 9.8", because that's pretty much the way they're being used.
*Of course, just about the only reason you'd need to differentiate is if the local gravity field is something other than 9.8 m/s/s, which would mean that you're dealing with space travel, which would mean that you're almost certainly using the metric system to begin with, and thus would have no need to use either pound-mass or pound-force for anything.

Thank you for answer, now I understand uses of units here.

IMHO this answer further perpetuates that mass and force are the same thing -- kg is N/9.8.  This is a terrible habit.  Force and mass are not the same thing at all.
And don't fool yourself that the aerospace industry works in the metric system.  If you're going to work in aerospace, you need to be comfortable with a lot of strange combinations of units.

@RobMcDonald I did mention that confusing mass and force is sloppy, but it's what we have. You do have to think of kg as N/9.8, because that's what everyone else is doing, as illustrated by the OP's example. The choice is either to be pedantic or to be practical. While I do agree that using the proper units would be better, that's just not the world we live in. Also, I've never seen any aerospace company use anything other than SI since before the turn of the century.

@hiddenWindshield I agree that society at large doesn't know the difference between mass and force -- and that we have to accept that units of pound-force, pound-mass, and frankly, kg-force exist.  And when kg-force is used, the user usually doesn't know the difference and they drop the -force.  I am not a theoretical zelot ignoring the real world.  I just think the advice "If you get confused, just think of "kg" as "newtons divided by 9.8", because that's pretty much what they are." goes too far and in fact perpetuates the problem.

@RobMcDonald I wasn't intending to offer general advice, I was speaking specifically to the OP's original question. I've edited my answer slightly to make that a little more clear.

The other answers are correct in that they describe Lift and Weight as Forces -- that should be measured in Newtons or pounds-force (lbf).
Mass is not the same thing as force (and must not be confused).  In an engineering context, we should use a consistent set of units.  This means that mass is typically described in kilograms or slugs.  People frequently use the pound-mass (lbm), but you must convert away from that when performing calculations.
What these two descriptions leave out is a definition of force and mass -- a lot of people unfortunately don't understand how they are different.
Force is an ability to change the momentum of an object.  How much oomph.
Mass is the quantity of matter in an object.  How much stuff is there.
As it turns out, objects resist changes in momentum in proportion to their mass.  F=m*a.  So a force is a measure of changing motion and mass is a measure of an object's tendency to resist its motion being changed.
When you understand how different these quantities are, it is a lot harder to confuse them.  Force and mass are very different things.
There are other cases where we use similar (or the same) units to measure things that are very different -- for example work (units of force*distance, say N m or ft lbf) which has the same units of torque.  Of course, work and torque are not the same thing.

Thank you for your answer. I aware that mass is not force. What I've found is that density is written by slug/ft^3 so the equation L=W in lb holds with out multiplying coefficient of gravitational acceleration. I was confused because I'm usually dealing with SI unit.
Maybe it is convention to write lbf just lb like 1g*kg = 9.8N = 1kgf so usually say 1kgf = 1kg in daily conversations.

While doing my degree in aerospace engineering in Germany, I don't think I've ever come across an actual usage of kilopond (kilogram-force), except for literature from before about 1970. When using kg, it is obvious that it's a mass, and you multiply by 9.81 m/s^2 to get the weight (on Earth) of that mass in N, and then use that value in those calculations that require an aircraft's weight. I'm not sure how other countries handle it, but I can't recall seeing it in France either.

I agree that kgf is commonly used -- but never written out.  It is not used in technical settings, but instead in ordinary life.  1kgf=1kg in daily use is the same thing as 1lbf=1lbm.  When I do technical work, I try to avoid lbm -- but I still label lbf (not lb) wherever possible.  The mass unit seems to be where the trouble always arises.  If you're working lbf, in, s, then the consistent mass unit is sometimes called blob, slinch, slugette, or snail...  Similarly, if you want to work in N, mm, s then mass is in tonne and density t/mm^3

Thanks for mentioning slugs!

Properly speaking, weight is the force of gravity felt by an object, and units of force (pounds-force or newtons) should be used. Both colloquially and in engineering, weights are often given in kilograms-force, which is equal to the force a 1 kilogram object feels when it is in a $1g$ gravitational field.
The pound is legally a unit of mass (it is defined as an exact number of kilograms, not newtons), but the American system has no equivalent definition for force and "pound" is often used unqualified for both a force and a mass.
Theoretically, an airplane is ever-so-slightly lighter at a higher altitude and so requires less lift. In practice, this difference is so minor as to be negligible for everyday purposes- it's less than $1\%$ everywhere that planes regularly go.

Thank you for answer, now I understand uses of units here.

The pound is a unit of mass, the pound-force is the unit of force.


text:
500, 400, 300, 200, 100 numbers said by planes computer
When landing what do the numbers said by the plane's computer mean? They are sometimes followed by retard, retard.   Is this the altitude or is this the feet away from the landing point on the runway?  Is there a way to know exactly where the plane should land on the runway?  I am a complete newbie but love the airplane vids on youtube. Thanks.

Welcome to Aviation.SE! I would recommend to ask your last question separately since it's not directly related to the other questions. Have a look at the tour to see how the site works.

Those are height callout, typically based on radar altitude, therefore measuring your height above ground. E.g. for the Airbus A320:

Automatic Call Out
FWC generates synthetic voice for radio height announcement below 2500
feet. These announcements come through the cockpit loudspeakers even
if the speakers are turned off.
Predetermined call out
The altitude call out uses the following predetermined threshold:

[...]
RETARD announcement
The loudspeaker announces RETARD at 20 feet or at 10 feet if
autothrust is active and one autopilot is in LAND mode.

(Airbus A320 FCOM - Navigation - Radio Altimeter)
For more information on RETARD, see Why does the A350-900 call out to retard at 20 feet above the runway?
The call outs are not always based on radio altitude. E.g. on the Boeing 737 NG the 1000 and 500 call outs are based on barometric altitude above landing field elevation (see this answer for details).

https://www.skybrary.aero/articles/altitude-flight-level-and-height

Why, of course it's assessing the pilot's IQ and medical condition!...
...Either that, or it's calling out the altitude in feet, followed by an instruction to retard thrust levers to idle, which is required for either wheel brakes only stop or thrust reverser deployment.
Most pilots choose to go with the second version, which also allows them to fly again.

The numbers are the radio altimeter call out - it's a very precise way of measuring height above ground, and the callouts are there to aids the pilots with their situational awareness as they approach the ground.
'Retard' is an airbus specific callout which is instructing the pilots to reduce thrust, as they approach the runway

The 500, 400, 300, 200, 100, Retard, retard is calling out the altitude. Retard means that the landing autoreverse and spoilers are on.

"Retard means that the landing autoreverse and spoilers are on." - I disagree... The "retard" callout occurs before touchdown, so it cannot be for either of those. Here is an explanation (by me) of the retard callout.

“Retard” does not mean anything is on, it is a reminder that the thrust is not yet set to idle and needs to be set to idle for the spoilers to activate.

There is no such thing as auto-reverse. Spoilers and brakes are automatic based on weight on wheels and thrust setting if armed, but reverse thrust is always actuated directly.


text:
Should I Get My Vision Corrected Prior to 1st Class AME?
I have 20/30 vision in both eyes. I don’t have glasses or a prescription for corrective lenses. 1st class AME requires correctable to 20/20. I need to obtain a 1st class certificate for an aviation scholarship. Do I need to get corrective lenses (to get to 20/20) before I have my evaluation? My optometrist is booked for the next six months. I’m wondering if I should schedule the AME or not.

Is this for the U.S. / FAA rules? Or elsewhere?

Almost certainly yes - the eyesight test is part of the assessment, and you'll be expected to demonstrate your corrected and uncorrected vision at the time.
If you fail, then you may not get given your medical. Specifically here in the EU, your medical (if necessary) will be marked that you must wear glasses for it to be valid. Again, your AME can't very well include this if you don't have glasses at the time.


text:
How to measure fuel level from uneven tank?
If you measure the fuel level with a dipstick on the C172 I fly, you can get two different readings. This is because directly under the filler cap opening there is a cuboid-shaped sensor. Obviously, measured from the top of the sensor, you get a much smaller value than if you lower the dipstick to the side of the sensor. (4 USGal difference)
Which value is correct?

The bottom of the tank is flat so you want to make sure the dipstick is touching the bottom. In any case, if you want accurate readings, it's not sufficient to just dip the fuel and estimate the level between the bottom and top of the opening, because if the tank is sloping, or is tapered, or is irregular, it doesn't give you an accurate reading.
Absent a commercial dipstick that is custom graduated for that tank, your best bet is to mark off your own dipstick by draining the tank through the gascolator, then filling it in 1 or 2 gallon increments and marking off the levels yourself. I've always done that with the airplanes I've flown regularly.